In recent years, to realize a storage apparatus capable of operating at a high speed, it has been proposed to use a plurality of solid state drives (SSDs) instead of hard disk drives (HDDs). In SSDs, unnecessary data is deleted in units of blocks according to particular timing (hereinafter, this process of deleting unnecessary data will also be referred to as garbage collection).
For example, garbage collection may be performed during a period in which a system using an SSD is in an idle state.
Japanese Laid-Open Patent Publication No. 2002-32256 and Japanese Laid-Open Patent Publication No. 2004-178417 disclose related techniques.
However, in the storage apparatus including a plurality of SSDs, an internal process such as garbage collection performed in the SSDs may cause a reduction in performance. When a plurality of SSDs are arranged in a “RAID 1” configuration which is one of configurations according to. Redundant Array of Inexpensive Disks (RAID) standards, its expected performance is equal to that achieved by a storage apparatus including a single SSD. However, performance achieved actually may be worse than that achieved by the storage apparatus including the single SSD.
For example, expected performance of a storage apparatus including a plurality of SSDs in a “RAID 1” mirror configuration may be about 70% to 80% of that achieved by a storage apparatus including a single SSD. This reduction in performance may be caused by response delays that may occur at different timing among the plurality of SSD in the “RAID 1” configuration. The response delays may be caused by internal processes such as garbage collection that may be different in terms of timing. That is, in the storage apparatus including the plurality of SSDs arranged in the “RAID 1” configuration, when an internal process is being performed in a first one of the SSDs, a second one of the SSDs waits for the internal process in the first one of the SSDs to be completed, and a response is not allowed until the internal process in the first one of the SSDs is completed. On the other hand, when an internal process is being performed in the second one of the SSDs at different timing, the first one of the SSDs waits for the internal process in the second one of the SSDs to be completed and a response is not allowed until the internal process in the second one of the SSDs is completed. Thus, a greater response delay occurs than may occur in the storage apparatus including the single SSD, which results in a reduction in performance.
Note that in a case of a storage apparatus including a plurality of HDDs which are equal in type and storage capacity, internal processes in the HDDs do not cause a reduction in performance. That is, the reduction in performance occurs only when a storage apparatus includes a plurality of SSDs.
The situation described above occurs not only due to garbage collection but a similar situation may occur due to another type of internal process peculiar to SSDs such as a snapshot process which is a process of capturing a data image. Also note that the situation described above occurs not only when a plurality of SSDs are arranged in the “RAID 1” configuration, but a similar situation may also occur in other RAID configurations such as “RAID 5”.